install tensorflow; caffe2 as runtime has issues with libc

Author: guschmue

.travis.yml

@@ -27,7 +27,7 @@ install:
   - printenv
   - conda config --set always_yes yes --set changeps1 no
   - conda install -c conda-forge protobuf numpy
-  - cd $TRAVIS_BUILD_DIR && pip install onnx pytest pytest-cov pytest-runner graphviz
+  - cd $TRAVIS_BUILD_DIR && pip install onnx pytest pytest-cov pytest-runner graphviz tensorflow
   - conda install -c caffe2 caffe2
   - cd $TRAVIS_BUILD_DIR && export PYTHONPATH=pwd